Cocaine seeking behavior and relapse have been linked to impaired potentiation and depression (LTP and LTD) at excitatory synapses in the nucleus accumbens (NAc), but the mechanism underlying this process is poorly understood. We show that, in the rat NAc core, D-serine is the endogenous coagonist of synaptic N-methyl D-aspartate receptors (NMDARs), and its presence is essential for NMDA-dependent LTP and LTD. NAc core slices obtained from cocaine-treated rats after 1 day of abstinence presented significantly reduced D-serine concentrations, increased expression of the D-serine degrading enzyme, D-amino acid oxidase, and downregulated expression of serine racemase, the enzyme responsible for D-serine synthesis. The D-serine deficit was associated with impairment of LTP and LTD that was restored by slice perfusion with exogenous D-serine. Furthermore, in vivo administration of D-serine prevented behavioral sensitization to cocaine. These results provide evidence for a critical role of D-serine signaling in synaptic plasticity relevant to cocaine addiction.